The present invention relates to an oscillator circuit, and more particularly to an oscillator circuit including an LC resonant tank, a differential comparator, and a start-up circuit.
FIG. 1 illustrates a simple LC resonant circuit comprising a capacitor C.sub.1 and an inductor L.sub.1. When this circuit is provided with power, such as by power supply PS, the capacitor C.sub.1 is charged.
Once capacitor C.sub.1 is charged and the power supply is removed, capacitor C.sub.1 acts to discharge into inductor L.sub.1, and the energy stored in capacitor C.sub.1 is converted to the magnetic field associated with inductor L.sub.1. When capacitor C.sub.1 is discharged current flow will come to an end. However, with nothing to sustain it, the magnetic field of inductor L.sub.1 will begin to collapse converting its energy back into electricity and recharging capacitor C.sub.1. During this time period, current flows around the circuit opposite its previous direction. With capacitor C.sub.1 again charged, the circuit is back to its original state, except that a small amount of energy will have been lost, due to inductive heat loss or other imperfections in the circuit. The cycle of charging/discharging repeats continuously until all the energy has been lost. The loss of energy is represented by the decaying output signal waveform of FIG. 2.
It is to be noted the circuit generates a frequency of oscillation which is constant, and that each combination of inductor L.sub.1 and capacitor C.sub.1 has its own resonant frequency, and altering the values of the elements alters the rate of oscillation.
In order to maintain the output signal from dying out, an amplifier AMP is added with a feed-back network FB, to produce feed-back voltage, such as shown in FIG. 3. If the phase shift through amplifier AMP and feed-back circuit FB is correct, the feedback signal at point X will be exactly in-phase with the signal driving the error terminals of amplifier A, so that the phase shift around the entire loop is 0.degree..
The loop gain of the amplifier circuit is an important consideration in configuration of an oscillator circuit. If the loop gain is less than unity, the output signal will die out because there will not be enough voltage being returned. On the other hand, if the loop gain is greater than unity, the loop gain voltage is greater than input voltage and the output voltage builds up such as shown by the waveform of FIG. 4. Finally, when loop gain equals unity, there will be no change in the output voltage and a steady state output will exist, such as depicted by the waveform of FIG. 5.
In a practical oscillator, the value of loop gain is greater than unity when the circuit is first energized. A small starting voltage is applied to the terminals of an amplifier, and the output voltage builds up. After the output voltage reaches a desired level, the value of the output gain automatically decreases to unity and the output amplitude remains constant.
The starting voltage of an oscillator is built into resistors of the oscillator circuit. Every resistor generates noise voltages, these voltages are produced by the random motion of electrons in the resistor. Thus, each resistor acts as a small voltage source producing essentially all frequencies.
With regard to an oscillator circuit such as shown in FIG. 3, when power is first applied to the system, the only signals in the system are noise voltages, which have very small amplitudes. The noise signals are amplified and appear at the output terminals. The amplified noise is used to drive the feedback circuit, usually a resonant circuit. The feedback voltage FBv will be maximum at the resonant frequency of the feedback circuit FB. In other words, the amplified noise is filtered so that only one sinusoidal component returns with exactly the right phase for positive feedback.
There are two common ways for loop gain to decrease to unity. First, the increasing signal will eventually force the output stage to clip any additional increases in output. When this happens, the value of gain decreases. The harder the clipping, the lower the voltage gain. In this way, the gain decreases to whatever value is needed to make loop gain equal to unity.
Another manner of limiting gain is to include an element into the feedback circuit to reduce the gain of the feedback circuit. Often, this element is a non-linear resistor that reduces gain when the output signal has reached the desired value. In this way, the oscillator automatically makes loop gain equal to unity after the oscillations have built up.
A widely used type of oscillator is known as a linear oscillator. A linear oscillator has a proportional output response to an input voltage. A drawback with linear oscillators is that the start-up time from when the noise is first applied in the system, to the time when a sufficient oscillation has been built up to reach a steady state, is unpredictable. This unpredictability exists since there are dependencies upon the size of the initial noise level and the amount of gain within the amplifier. For example, if the noise initially produced in the oscillator is very small, it will take a longer time than if the initial noise is larger. Also, the different gains of a linear amplifier will also cause unpredictability in the time it takes to reach a steady state.
A less well known oscillator is a non-linear oscillator. In a non-linear oscillator, the amplifier is configured as a comparator circuit. A distinction between linear and non-linear oscillators is that, unlike a linear oscillator, the non-linear oscillator requires some sort of start-up mechanism, i.e. it does not have an automatic self-starting capability. Examples of non-linear oscillators are described in U.S. Pat. Nos. 4,833,427 to Leuthold et al., 4,879,512 to Leonard et al., 4,617,534 to Lill, and 4,560,953 to Bozio. Each of these are class of oscillator which includes a comparator, resonant tank and start-up mechanism. The cited patents are generally directed to different schemes for applying noise to the oscillator system.
The U.S. Pat. No. 4,560,953 describes a start-up circuit which includes a starting capacitor C.sub.2 wherein noise is induced on one of the inputs of the comparator and the comparator reacts to signals and sends back a feedback signal to the resonant tank. The resonant tank selects the correct frequency from that signal and starts a circulating current. On starting, the starting capacitor C.sub.2 is connected to the non-inverting input and to the power supply. Therefore on power start-up, current goes through the starting capacitor C.sub.2 which causes a voltage drop across the resonant tank circuit (C.sub.1, L.sub.1) and the voltage drop creates a start-up pulse. A drawback of this type of circuit is that if power is applied slowly, the level of current may not be sufficient to begin oscillation. This drawback can be detrimental in certain environments, such as inductive sensing, where it is not known whether power will be applied to the system gradually or quickly. It is, therefore, considered desirable to have an oscillator which can start oscillations irrespective of whether a gradual or quick application of power is made to the oscillator.
Thus, the present invention is directed to an inductive proximity sensor oscillator which will reach a desired start-up in a fast, efficient manner irrespective of whether power applied to the system is gradually or quickly applied. The desired operation of the oscillator being accomplished by the use of a negative feedback start-up network.
Therefore an object of the present invention is to provide a non-linear oscillator circuit which includes a start-up mechanism formed in a simple and reliable manner.